The invention relates to monitoring a connection to a user terminal in a telecommunications system.
The present invention can be used in different telecommunications systems, such as Universal Mobile Telecommunications System (UMTS). In the following, the invention is described by way of example with reference to the UMTS, more specifically to the UMTS system being specified in the 3rd Generation Partnership Project 3GPP without restricting the invention to it.
FIG. 1 illustrates an example of the network architecture of an UMTS system. In the figure, unbroken lines between various network elements represent signalling and data interfaces whereas broken lines between network elements represent mere signalling interfaces. Main parts of the system are a radio access network providing access to user terminals UE (User Equipment) and a core network. In the example of FIG. 1, the core network comprises a Serving GPRS (General Packet Radio Service) Support Node SGSN, a GGSN (Gateway GPRS Support Node), an HSS (Home Subscriber Server) and a CSCF (Call State Control Function). The support nodes SGSN and GGSN are interconnected by a backbone network, such as an IP/ATM (Internet Protocol/Asyncronous Transfer Mode) network. It should be noted that the functionalities of the SGSN and the GGSN can also be physically combined into the same network node, in which case the operator's backbone network is unnecessary. Logically, however, the nodes are separate nodes. Core networks of another type may comprise other network elements. The core network can be connected to external networks, such as IP networks and PSTN/ISDN networks, as illustrated.
The CSCF controls call establishment and is responsible for routing calls, and comprises, for example, a function corresponding to a switching function in the intelligent network. The CSCF provides IP telephony services with end-to-end control. Signalling associated with the IP telephony, such as H.323 and SIP (Session Initiation Protocol), terminates at the user equipment and the CSCF. In other words, the CSCF is the network node in which IP telephony user equipment UE is registered and via which the signalling is transferred. The CSCF comprises IP telephony call state models, which are used for controlling call establishment with other network nodes. The CSCF can also communicate with IP telephony application servers (not shown in FIG. 1). The CSCF comprises a subscriber database, which logically corresponds to a visitor location register in the GSM system. The CSCF is responsible for producing both telephony billing information and service billing information.
For a user terminal UE, the core network GPRS interface comprises one or more individual PDP (packet data protocol) contexts which describe the packet data address the UE can use to send and receive data packets when the PDP context is active. Thus, the PDP context can be seen as a connection. The PDP context defines different data transmission parameters, such as the PDP type (e.g. X.25 or IP), PDP address (e.g. IP address), quality of service QoS, access point name APN and NSAPI (Network Service Access Point Identifier). The IP telephony is in practice invisible to the elements of the core network. For the support nodes SGSN and GGSN, the IP telephony is only a PDP context with certain service quality requirements. The signalling associated with the IP telephony terminates at the user equipment and the CSCF, so there is no need for the SGSN or GGSN to understand it.
The problem in the above arrangement is that the control and the media i.e. transfer of user data (such as voice) are distinguished, wherefore, during a connection, the controlling entity, here a CSCF, has no actual idea about the availability or state of the connection to the user terminal UE, unless it tries to send some signalling message by using the connection. Also, the media part, here an SGSN/GGSN, has no means for informing the CSCF in case there was a notable change in the availability of the connection. Once the terminal goes out of coverage, the following is expected to happen: the radio access network discovers that the radio link is not OK and informs the SGSN. Relase of the Iu interface, etc., is performed as defined in the standards. In the user terminal, the radio or GPRS part may inform the application that the connection was lost and the application in terminal goes to idle state. The CSCF will continue assuming that the connection is active until it needs to send some (e.g. an SIP) message to the user terminal; once the failure of the delivery is notified by the GPRS part to the CSCF, the CSCF may conclude that the connection must be terminated and it will go to idle. Such a message might be caused e.g. by the other party of the connection, the user of which presses the end call button of the user terminal upon noticing that the connection to the first party has been lost. The problem is, however, that the CSCF may be in a wrong kind of state for a long time, thus causing inconsistent charging, for example. Also, a case where, for example, CFNRc (Call forwarding on not reachable) is activated but no CFB (Call forwarding on busy) is activated would result in wrong actions in the CSCF. In the GSM system, for example, no similar problem exists since the control and the media are handled by a single entity, i.e. a Mobile Switching Centre (MSC).